Lithium metal polymer batteries are typically built as large format batteries of 20 kWh or more for use in electric vehicle, in stationary applications for back-up to ensure continuity to applications that cannot afford a grid power outage such as telecommunication stations, data centers, etc., or to provide alternate power source for peak shaving purposes in industrial or residential buildings.
Lithium metal polymer batteries consist of one or more elementary electrochemical cell laminates comprised of thin layers, each laminate including an anode or negative electrode made of a lithium or lithium alloy metallic sheet layer, a cathode or positive electrode film layer made of an electroactive active material in a polymer-salt binder spread onto a metallic current collector, and a solid electrolyte comprising a thin layer of a polymer and a lithium salt mixture separating the positive and negative electrodes and providing ionic conductivity between the electrodes. More specifically, the positive electrode consists of electrochemically active material particles, an electronically conductive additive and a solid polymer electrolyte which acts as a binder and provides the required ionic path between the electrochemically active particles and the adjacent solid electrolyte separator.
It is well established that the performance and service-life of a lithium metal polymer batteries are significantly improved by maintaining the layers of the electrochemical cell laminates in a state of compression. In a state of compression, the ionic migration at the various interfaces is improved and the potential dendrite growth on the surfaces of the lithium metallic sheet is significantly reduced. The thermal conduction characteristics of a stack of electrochemical cells are also significantly improved when forced contact between adjacent cells is maintained. Improved performance and service-life has been achieved by maintaining pressure on the laminates with a mechanical pressure system comprising a series of spring-type elements which apply compressive forces on the opposing surfaces of the laminates throughout the battery cycle (charge-discharge).
The necessity of including a mechanical pressure system in the design of a lithium metal polymer battery has limited the configuration of such battery to a prismatic layout consisting of a plurality of laminates stacked one on top of the other to form a prismatic electrochemical cell and stacking a plurality of prismatic electrochemical cells one on top of the other to form a large format battery. Otherwise the lithium metal polymer battery could be made of a single laminate spiral rolled to form a cylindrical battery or flat rolled to form a flat wound prismatic battery but the mechanical pressure system would be much more complex and difficult to assemble into a cylindrical or semi-cylindrical container or casing.
The mechanical pressure system requires spring-type mechanism because cyclical volume changes occur in the charge and discharge cycles of the lithium metal polymer electrochemical cell. The volume of an electrochemical cell expands and retracts during charge and discharge cycle respectively due to the migration of lithium ions between the lithium metal anode and the lattice structure of the cathode material. During the charge cycle, the lithium ions migrate out of the lattice structure of the cathode material and are plated onto the surfaces of the lithium metal sheet of the anode thereby increasing the thickness of the anode and therefor its volume by as much as 8%. In the discharge cycle, the lithium ions plated onto the surfaces of the lithium metal sheet of the anode migrate back to the cathode and are inserted back into the lattice structure of the cathode material thereby reducing the thickness of the anode and its volume by the same 8%.
Lithium metal polymer batteries uses a solid polymer electrolyte rendering this technology extremely safe. However, to obtain optimal ionic conductivity and therefore optimal performance, the electrochemical cells must be heated to temperatures of 60° C. to 80° C. Lithium metal polymer batteries therefore include a heating system to maintain the battery at a nominal temperature of about 40° C. and to rapidly raise the temperature of the electrochemical cells to between 60° C. and 80° C. in operation. The rise in temperature of the electrochemical cells also results in thermal expansion of the volume of the cells by an additional 3%.
In modules or batteries comprising numerous thin-film electrochemical cells in a stack configuration, the volume change resulting from ionic migration and thermal expansion is compounded such that the overall volume change is significant and must be accommodated.
In order to accommodate these compounded variations in electrochemical cell volume resulting from charge and discharge cycling of a grouping of electrochemical cells, an active mechanical pressure system comprising spring-type elements adjacent to the walls of the container or casing is typically used to absorb these large variations of volume while maintaining an evenly distributed pressure onto the electrochemical cell stack throughout the volume expansion and volume reduction during charge/discharge cycling. For large battery applications, the active mechanical pressure system typically comprised of a plurality of metal springs applying pressure against a metal plate which can generate the necessary compressive force throughout the volume expansion and reduction, and may also include spring inserts located between adjacent electrochemical cells within the cell stack to enhance distribution of compressive forces within the cell stack.
An active mechanical pressure system as described above is bulky and represents a weight penalty which by default decreases the energy density (W/Kg) of the lithium metal polymer battery. Furthermore, as described above, the mechanical pressure system limits the configuration of a lithium metal polymer battery to a prismatic layout otherwise the lithium metal polymer battery could have a cylindrical configuration or a flat wound prismatic configuration.
Thus, there is a need for a lithium metal polymer battery designed and assembled without a mechanical pressure system to maintain the electrochemical cells of the battery under pressure.